When a full high-definition television (HDTV: High Definition Television, 1080×1920 pixels) receiver enlarges an image signal with resolution lower than that for the HDTV and displays an image thus obtained, the image becomes blurry. Similarly, when an image represented by an image signal with resolution for the HDTV is enlarged to an image with higher definition (for example, 4K resolution of approximately 4000×2000 pixels), the image becomes blurry. As such, a conventional television receiver performs edge compensating for sharpening rise and fall of a video signal corresponding to an outline portion of an image to be displayed. In edge compensation, a high frequency component of an input image signal (a luminance signal) is extracted, amplified, and then added to the input image signal, thereby improving visual image quality.
Here, when sharpening processing is carried out on a high frequency component in a horizontal direction and a high frequency component in a vertical direction of an image, a phenomenon in which an oblique line of the image subjected to the sharpening processing appears to glitter occurs, which is likely to become a problem especially in sharpening processing employing nonlinear processing for generating a high frequency component exceeding a Nyquist frequency.
FIG. 19 is a diagram illustrating a configuration for consecutively performing, in the vertical direction and in the horizontal direction, the sharpening processing for generating the high frequency component exceeding the Nyquist frequency. FIG. 20 are diagrams illustrating a frequency component of a signal at each stage. FIG. 20A illustrates a frequency component of an input image signal Sin of a digital image having a sampling frequency fh in the horizontal direction and a sampling frequency fv in the vertical direction. The digital image has a Nyquist frequency fh/2 in the horizontal direction and a Nyquist frequency fv/2 in the vertical direction and, as illustrated in the figure, there is no frequency component in a range exceeding the Nyquist frequency. When the sharpening processing is carried out on the input image signal Sin in the vertical direction, in a signal S1 thus obtained, as illustrated in FIG. 20B, the frequency component is generated in a wide region exceeding the Nyquist frequency fv/2 in the vertical direction. When the sharpening processing is further carried out on the signal 51 in the horizontal direction, in an output image signal Sout thus obtained, as illustrated in FIG. 20C, the frequency component is generated in a wide region exceeding the Nyquist frequency fh/2 in the horizontal direction. As illustrated in the figure, regions at four corners of the frequency component of the output image signal Sout, i.e., regions at a high frequency in both the horizontal direction and the vertical direction are subjected to the sharpening processing in the horizontal direction and in the vertical direction in an overlapping manner, whereby the glitter of the image is emphasized.
In order to clear such glitter, a technique having a two-dimensional filter disposed at a preceding stage of horizontal sharpening processing and vertical sharpening processing has been proposed (see PLT 1).